Kuwata
Chemistry 115
Fall 2008
Accelerated General Chemistry
Chapter 3: Molecular Geometry (in Atkins and Jones)
•
Goal: Convert a Lewis structure of a species into its three-dimensional structure:
o Shape (an overall description)
o Bond Angles
o [Trends in bond lengths]
•
Realize goal with Valence Shell Electron Pair Repulsion (VSEPR) Theory
o Electron groups around an atom repel each other
o Therefore, maximize the angles between electron groups
•
Two key terms (not explicitly defined in Atkins and Jones):
o Coordination Number (CN) = the number of atoms (also known as substituents)
bonded to a given atom
o Steric Number (SN) = the number of electron groups around a given atom
SN = CN + (the number of lone pairs) + (the number of unpaired electrons)
o CN and SN can be determined for any atom in a species (not just a central atom)
•
Need both SN and CN to fully define a molecule’s shape
VSEPR Theory for Compounds with Normal Valences
(most relevant to Organic Chemistry next year)
You must memorize all of the following ideal bond angles,
electron group arrangements, and shape names
SN = 2
Ideal Bond Angle = 180º
Electron Group Arrangement: linear
Possible CN’s
Lewis Structure e.g.
Cl
CN = 2
Be
Cl
or
Cl
Shape
Cl
Be
Shape Name
Cl
linear
Be
180°
Cl
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Kuwata
SN = 3
Chemistry 115
Ideal Bond Angle = 120º
Electron Group Arrangement: trigonal planar
Possible CN’s
Lewis Structure e.g.
CN = 3
H B H
H
CN = 2
Cl
Sn
Shape
H
Shape Name
H
B 120°
H
Cl
(large Group 14 can have
less than an octet)
SN = 4
Fall 2008
Cl
Sn
Cl
trigonal planar
bent
(or angular)
(names based on
atom locations only)
Ideal Bond Angle = 109.5º
Electron Group Arrangement: tetrahedral
Possible CN’s
Lewis Structure e.g.
CN = 4
H
H C H
H
CN = 3
Shape
109.5° H
H C
H
H N H
H
H
H O H
CN = 2
N
H O
H
tetrahedral
H
H
H
H
H O
or
Shape Name
trigonal pyramidal
bent
(or angular)
(but how similar is it
to SnCl2?)
If for a given atom (1) SN = CN and (2) all substituents are identical, the actual (experimental)
bond angles will be the same as the ideal bond angles (by symmetry).
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Kuwata
Chemistry 115
Fall 2008
VSEPR Theory for Hypervalent Compounds
Shape Names
(ideal bond angles)
Shape Names
(ideal bond angles)
Ideal Bond Angle = 90º
Electron Group Arrangement:
octahedral
Ideal Bond Angles = 90º, 120º
(180° for linear)
Electron Group Arrangement:
trigonal bipyramidal
You are not required to memorize the shape names for SN = 5 and SN = 6. However, you
should be able to explain the shapes and bond angles.
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